This invention relates generally to apparatuses and methods for joining materials and the products created by the joining methods. More particularly, the invention relates to apparatuses and methods for welding pieces of industrial textiles, such as thermoplastic materials or thermoplastic coated fabrics, together. The invention further relates to joining, by welding or with adhesive tape, fabrics intended for use in the garment industry, particularly the outdoor technical clothing industry, without employing traditional sewing techniques to join fabric pieces.
The industrial textile industry is based on the availability of a variety of thermoplastic extruded sheeting and thermoplastic coated fabrics. These materials are used to make a wide range of products such as, for example, inflatable boats, hot air balloons, covers for outdoor structures, geo-membranes for lining toxic waste sites, awnings and tents, outdoor banners, artistic fabric sculptures, liquid transportation containers, dry bags, and waterproof storage sacks. The products are generally lightweight, can be folded to a small size when not in use, have coatings that are generally impervious to industrial chemicals, and can be purchased in a wide range of colors, textures and fabric weights. Such fabrics generally come in approximately 5 to 12 foot wide rolls and thus must be cut into the required pattern pieces before being joined together to make the completed product.
There are three basic methods by which pieces of coated fabric can be joined together to create a mechanical bond as well as water-tight and gas tight seams: traditional sewing followed by applying waterproof tape to the seam, gluing, and heat sealing (also called welding). Fabrics coated with certain rubber based coatings, such as Hypalon (manufactured by DuPont) can only be glued or sewn. Most of the newer coatings including polyurethane, polyvinylchloride, polypropylene, and polyethylene can either be glued or welded. However, gluing can be very labor intensive and further is subject to strict scrutiny from the Occupational Safety and Health Administration (xe2x80x9cOSHAxe2x80x9d) due to the volatile solvents that are employed during the gluing operation. Most gluing now takes place in countries other than the United States.
There are four main methods of heat sealing or welding in use: hot wedge, radio frequency (xe2x80x9cRFxe2x80x9d), ultrasonic, and hot air. In the hot wedge method, two fabric pieces are drawn across a hot iron (or wedge) and then are pressed together. This method is quite similar to the hot air process with only the heat delivery system being different. One disadvantage of this method is that the wedge can become contaminated with dirt and melted plastic which then reduces the amount of heat delivered to the seam. Further, hot wedge welders typically experience a hot section immediately after the beginning of the weld as the wedge accumulates excess heat when idle. Another disadvantage is that, since the heat energy must pass through a solid object to reach the seam, the maximum speed of the hot wedge welding process is limited by the thermal conductivity of the wedge.
The RF method is probably the most widely used approach for heat sealing. The RF welder is basically an antenna (the die) that is poorly matched to the amplifier, thereby producing a great deal of heat rather than radio waves between the antenna and the underlying plate. In practice, the two fabric pieces are laid on the plate. The die is then brought down, thereby pressing the two pieces together. The operator then initiates the welding process by pressing a pair of push buttons. The actual RF process takes from about 5 to 15 seconds, depending upon the thickness of the pattern pieces and the amount of RF energy available from the machine.
There are, however, several disadvantages to the RF method. RF welding is a slow process because the size of the die is limited by the available energy of the unit. Typical RF welding dies are about 1 to 3 feet in length and approximately 2 inch wide. There is also some concern about the operator""s health and safety as the operator is usually inches from an intense RF source which may be activated several hundred times in a typical shift. While RF health hazards have not been documented, it is known that stray RF energy from such machines can damage electrical equipment within approximately 50 feet of the machine and can light fluorescent fixtures located nearby. In addition, due to the die and plate arrangement, the RF method is typically limited to seams or joints that can be laid flat for welding. Three dimensional dies and plates are occasionally used, but are quite expensive and require a vacuum or other methods to hold the fabric in position as the die is applied. Further, the Federal Communications Commission (xe2x80x9cFCCxe2x80x9d) has become increasingly strict regarding emissions of stray RF energy from industrial sources. Because of the increasingly strict FCC regulations, new RF welding equipment can typically cost $80,000 or more.
Ultrasonic welding is a process that is like RF welding, with the exception of the energy source. Rather than using radio waves, ultrasonic welding uses sound waves that basically vibrate the fabric molecules until sufficient heat is generated to melt the coatings.
In general, hot air welding is much faster than other methods, can accommodate three dimensional patterns, and requires no dies or tooling. In a hot air welder, the flow of hot air that floods the seam is not subject to contamination, as with the wedge welder, and there is no initial drop off of heat at the beginning of the seam. Most fabricators want the speed of hot air technology, but have felt that it is difficult to obtain consistent results for many types of coated fabrics and also that it requires highly trained operators.
The typical rotary hot air welding apparatus uses hot air to join together two pieces of plastic coated fabric. The welder first injects a stream of hot air from a hot air nozzle between the two pieces of coated fabric. The temperature of the hot air can be set in the range of approximately 500 to 1350 degrees F. The fabric pieces are then pinched between and pulled through the apparatus by two drive wheels. The distance from the hot air nozzle and the pinch point between the two wheels is in the range of approximately 0.5 to 0.75 inch. The wheel speed determines how long the fabric is exposed to the hot air stream before it passes between the wheels. With a constant air temperature, the amount of heat energy delivered to the fabric is inversely proportional to the wheel speed; a faster speed decreases the exposure and vice versa.
Commercial hot air welders currently available on the market have a number of shortcomings. One shortcoming is the lack of accurate control of the speed of the two drive wheels. If the wheel speed varies from the required speed, then the amount of heat delivered to the seam will vary. Too much heat supplied to the weld results in burnt fabric while too little heat results in cold welds or unwelded fabric.
The problem of providing accurate wheel speed is compounded by the need to control both wheels independently. For some fabric patterns, especially patterns with curves, one wheel may need to run slightly slower or faster than the other wheel. Commercial hot air welders typically use a single DC motor with a variable speed (voltage) amplifier. The drive energy from the motor passes through a long series of chains and pulleys to the bottom drive wheel. The drive energy to the top drive wheel first passes through a variable diameter pulley transmission that provides adjustment for the relative wheel speed and then passes through a similar set of chains and pulleys.
This arrangement is fairly inaccurate and is not easily or consistently repeatable. With any particular speed setting, the actual wheel speed can vary with both the temperature of the amplifier and the motor windings and with the load on the motor. This is typical of a DC drive system in which there is no feedback to the motor.
Further, the variable diameter pulley that provides differential speed control is an inherently inaccurate mechanical device. The same differential speed setting is not repeatable between consecutive seams. In view of the inaccurate DC drive system and the variable speed transmission for the drive wheels, wheel speed adjustment and calibration are constant problems. These problems are particularly evident when thinly coated fabrics are being welded and where the amount of heat energy delivered to the seam must lie within a narrow range.
In addition, the inaccurate control of the wheel speed results in the two edges of the fabric being joined not xe2x80x9cin registration.xe2x80x9d In other words, at the end of the seam, one piece is shorter or longer than the other piece. Such an occurrence effects the overall quality of the product being made and is especially likely to occur when two different types of fabrics, with, for example, different elastic qualities, are joined together.
Thus, there is a need for a hot air welding apparatus and welding method that provides accurate control of the drive wheel speed and, consequently, accurate control of the amount of heat applied to the seam. There is a further need for a hot air welding apparatus and method that provides a differential speed setting that is repeatable between consecutive seams and that allows the drive wheel settings to be adjusted while the seam is being welded. In addition, there is a need for a hot air welding apparatus and method that joins the edges of two pieces of fabric xe2x80x9cin registration.xe2x80x9d
Further, generally, pieces of fabric which are joined for the manufacture of technical outdoor xe2x80x9cwaterproofxe2x80x9d type clothing are stitched together to form seams. This type of seaming requires that multiple holes be punched through the fabric pieces during the sewing/threading process. Thereafter, the seam must be treated so that it is waterproof. This two step process is inefficient because it requires that the stitches be inserted and that the newly made seam, and the requisite holes, be waterproofed in a second step. Further, the thread used to stitch the seam may rot or otherwise degrade over time because of, for example, exposure to ultra violet light, thereby weakening the seam. In laminated fabrics, each thread hole creates an untreated exposed edge where a one fabric layer may start to separate from another layer or from the waterproofing layer. Thus, traditional threading creates hundreds of points where delamination may easily occur, possibly resulting in a poorer quality product.
Therefore, there is a need for a method of seaming garments that does not require the use of traditional stitching/sewing techniques. In particular, there is a need to provide a method for seaming garments that are in the technical outdoor clothing industry, especially those with waterproof qualities, without sewing and without the need for putting needle holes in the waterproof fabric. Further, there is a need to join waterproof fabrics together in a single step in order to reduce labor costs.
The present invention provides a hot air welding apparatus and related method for accurately and repeatably controlling the drive wheel speed. To afford very precise control of the speed and relative rotation of the drive wheels at all times during the welding process, the present invention is provided with a computer control system and two high torque stepper motors, one for each drive wheel. The computer control system allows the speed and position of the drive wheels to be regulated with a very high accuracy. Further, the computer control system allows the stepper motors to be mounted very close to the drive wheels, thereby eliminating the extensive set of chains and pulleys that are inherent in the welders of the prior art and thus further reducing the inaccuracies brought about by the stretched chains and other elements of the drive chain. The computer control system allows the drive wheel settings to be adjusted while the seam is being welded and further includes an automatic ramp capability, a repeat mode, and a test strip mode. Further, the computer control system is capable of controlling the temperature and volume of air used in the welding process. The invention allows for the welding of fabrics with dissimilar thicknesses and stretch characteristics.
The invention further provides a method for joining fabric pieces, and garments seamed by such methods, particularly fabric pieces for technical outdoor waterproof clothing, without the need for traditionally sewn seams.
Many fabrics used in outdoor technical waterproof/breathable type clothing do not bond well, either to each other or with adhesives. However, traditional sewing is undesirable because holes are put in the waterproof fabrics during sewing. In addition, threads may degrade faster than the fabrics they join, weakening and reducing the life of the garment, and the perforations may weaken the fabric. Further, sewing and then sealing the threaded seams is a more expensive, labor intensive process than a one step seaming process. Material costs are also reduced by the elimination of thread and, in some cases, it also helps cut material costs by eliminating thread and possibly even seam tape if the fabrics are themselves weldable.
The main obstacles to overcome in the development of a stitchless seam in a technical fabric are the properties of the fabric itself. Most technical fabrics are made of nylon, which is not weldable because it melts and shrinks too much to allow the formation of a good bond at the seam. However, other thermal plastic materials, such as polyurethane, polyvinylchloride, polypropylene and polyethylene will bond well with heat.
Further, fabric weave also affects bonding. Most technical fabrics have a tight weave to aid in water repellency. However, a tight weave prevents an adhesive from permeating the fabric and forming strong bonds with the fabric fibers. Adhesive tapes may stick to nylon if the weave of the fabric is loose because the tape can encapsulate the individual fibers of the fabric, but loose weaves may be less water repellent.
To compound the problem further, most technical fabrics are coated or impregnated with a durable water-resistant solution (DWR). DWR""s are usually silicone based. Silicone is not a thermal plastic so it will not weld and adhesives will not adhere to it well.
These problems can be overcome if the fabrics are modified in one or more ways. For example, fabrics may be given thermoplastic characteristics. To accomplish this, weldable thermoplastic fibers may be included with the other threads which make up the fabric. Polyurethane threads will work well due to their good welding characteristics, elasticity and abrasion resistance, but other thermoplastics will also work.
Alternatively, fabrics with thermoplastic fibers having good bonding characteristics may be seamed with thermally activated adhesive tape. Though the fabric weave may be too tight for fiber encapsulation, exposed thermoplastic fibers in the fabric will still form a strong surface bond with various adhesives. The preferred seam in this situation is the butt seam. However, prayer, double butt, overlap and other seams will work as well.
Further, the DWR may be formulated to have thermoplastic properties so that it is compatible with the welding process. For example, a polyurethane base may be used instead of a silicone base.
Further, the use of adhesive tapes and modified water-resistant solutions or modified fabrics may be combined.
Alternatively, finished products may be coated with the DWR after the seaming process.
Another alternative is to use a DWR with a low evaporation temperature. The welding apparatus may be configured so that heat causes the DWR to vaporize and evacuate the bonding area the instant before the bond is formed. For example, a reflected blast of heated air that is escaping the point of the weld could accomplish this. Various heat sources may be fitted to the welding apparatus, including, but not limited to a specialized hot air nozzle, steam jet or laser.